The present invention relates generally to systems for providing supplemental oxygen to patients and, more particularly, to oxygen conserving devices for such systems.
Respiratory diseases, such as bronchitis and emphysema, cause patients to suffer from deterioration of lung function. Health care providers often prescribe supplemental oxygen to such patients so that they can inhale the supplemental oxygen along with ambient atmospheric air in order to maintain a sufficient oxygen concentration level in the blood stream. The supplemental oxygen is provided by a system that stores or generates the oxygen and provides it to the patient via a nasal cannula.
In early supplemental oxygen delivery systems, oxygen was delivered on a continuous flow basis, albeit at a low, fixed flow rate, throughout the entire breathing cycle to the nose of the patient by a tube which interconnected a source of oxygen with a nasal cannula. Although such systems were effective, oxygen was lost to the ambient atmosphere since the continuous flow of oxygen was provided to the patient's nose regardless if the patient was inhaling or exhaling.
In response to the waste of oxygen associated with the earlier prior art supplemental oxygen delivery systems, more efficient prior art systems and devices were developed and implemented for delivery of supplemental oxygen to patients. These devices included oxygen conserving features. Such devices include those which provide oxygen “on demand” to the patient. “On demand” systems deliver oxygen to the patient after the beginning of the inhalation interval of the breathing cycle while no oxygen is delivered to the patient during any portion of the exhalation interval of the breathing cycle.
Examples of such prior art supplemental oxygen delivery devices are presented in U.S. Pat. Nos. 4,462,398 and 4,519,387, both to Durkan et al. In the device of these patents, a control circuit responsive to a sensor operates a valve to supply pulses of oxygen through a cannula to a patient when negative pressure indicating the initial stage of inhalation is detected by the sensor. The sensor may be a pressure-to-electric switch/pressure transducer. The pulse of gas delivered to the patient can have a preselected pulse profile.
U.S. Pat. No. 4,686,975 to Naimon et al. also discloses a supplemental respiratory device wherein small pressure changes within an airway are monitored so that gas is only supplied during patient inhalation. The nasal cannula leads from the airway to the sensing means which takes the form of a pressure transducer.
In the devices of the above patents, and other similar prior art devices and systems, it is advantageous to apply a commercially available pressure transducer. Such transducers are available from Honeywell Inc. of Morristown, N.J. (Micro Switch model 24 PC), Sensym of Milpitas, Calif. (model SX) and other manufacturers. Such pressure transducers are readily available, inexpensive, small and reliable.
In order to reliably identify the onset of patient inhalation, the pressure transducer must sense a pressure in the cannula of approximately −0.1 cm of water at atmospheric pressure. Pressure transducers have an associated drift due to time, temperature and applied pressure (hysteresis). As a result, to achieve the necessary pressure sensitivity, the pressure transducer must be accurately calibrated or zeroed at atmospheric pressure. The pressure transducers of the above paragraph, however, do not feature a built in reference to atmospheric pressure. Accordingly, the problem exists as to how to efficiently calibrate or zero the pressure transducer of an “on demand” supplemental oxygen delivery device.
Some prior art supplemental oxygen delivery devices, such as the Pulsair/DeVilbiss OMS 20, have an adjustable pressure transducer by which the set triggering point may be manually adjusted. The set triggering point is the pressure at which the device is triggered to deliver oxygen. Such devices, however, require periodic adjustment to assure consistent triggering without “auto-cycling.” Auto-cycling occurs when the set trigger point is at or above atmospheric pressure.
Other prior art supplemental oxygen delivery devices, such as the AirSep Impulse Select, monitor an airway for a change in pressure instead of an absolute pressure level. This approach, however, carries with it the disadvantage that under some breathing conditions, the device might interpret a strong exhalation (decreasing expiratory flow) as the same as an inspiration (increasing inspiratory flow) as the slopes of the pressure profile waveforms for each are similar. As a result, the device may be triggered to deliver oxygen at an inappropriate time.
The pressure transducer of a supplemental oxygen delivery device may also be calibrated to an assumed atmospheric pressure when the device is powered on, as in the case of the DeVilbiss EX2000 device. Such an approach, however, may result in an incorrect reference pressure if the patient is breathing on the cannula when the device is turned on. An incorrect reference pressure causes the device to be difficult to trigger or the device may trigger, and thus deliver oxygen, at the wrong time.
Finally, prior art supplemental oxygen delivery devices such as the Transtracheal Systems DOC2000, DOC3000 and DeVilibiss EX2005 feature two valves. The first valve controls the delivery of oxygen to the patient. The second valve is used to vent the pressure transducer to atmospheric pressure while oxygen is being delivered to the patient. This allows the pressure transducer to be calibrated or zeroed to atmospheric pressure. While this approach is reliable, it has a high associated cost and power consumption because two valves are required.
Accordingly, it is an object of the present invention to provide a self-calibrating supplemental oxygen delivery system;
It is another object of the present invention to provide a supplemental oxygen delivery system that is reliable;
It is another object of the present invention to provide an “on demand” supplemental oxygen delivery system;
It is another object of the present invention to provide a supplemental oxygen delivery system that is cost effective to construct;
It is still another object of the present invention to provide a supplemental oxygen delivery system that is economical to operate.